Elevating SOX2 Downregulates MYC through a SOX2:MYC Signaling Axis and Induces a Slowly Cycling Proliferative State in Human Tumor Cells

SIMPLE SUMMARY: The ability of slowly cycling/infrequently proliferating tumor cells to evade treatment with therapies that target actively proliferating tumor cells represents a major clinical challenge. Previous studies established that high levels of SOX2 in both fetal and tumor cells restrict cell proliferation and induce a slowly cycling state. However, the mechanisms through which elevated SOX2 levels inhibit tumor cell proliferation have not been identified. The studies presented here set out to determine the mechanisms through which SOX2 elevation restricts tumor cell proliferation. We demonstrated that elevating SOX2 decreases the expression of MYC and MYC target genes. We also determined that the downregulation of MYC is a critical mechanistic step necessary to maintain survival in the slowly cycling state induced by elevated SOX2. Altogether, our studies uncover a novel SOX2:MYC signaling axis and provide important insights into the molecular mechanisms through which SOX2 elevation induces a slowly cycling proliferative state. ABSTRACT: Slowly cycling/infrequently proliferating tumor cells present a clinical challenge due to their ability to evade treatment. Previous studies established that high levels of SOX2 in both fetal and tumor cells restrict cell proliferation and induce a slowly cycling state. However, the mechanisms through which elevated SOX2 levels inhibit tumor cell proliferation have not been identified. To identify common mechanisms through which SOX2 elevation restricts tumor cell proliferation, we initially performed RNA-seq using two diverse tumor cell types. SOX2 elevation in both cell types downregulated MYC target genes. Consistent with these findings, elevating SOX2 in five cell lines representing three different human cancer types decreased MYC expression. Importantly, the expression of a dominant-negative MYC variant, omomyc, recapitulated many of the effects of SOX2 on proliferation, cell cycle, gene expression, and biosynthetic activity. We also demonstrated that rescuing MYC activity in the context of elevated SOX2 induces cell death, indicating that the downregulation of MYC is a critical mechanistic step necessary to maintain survival in the slowly cycling state induced by elevated SOX2. Altogether, our findings uncover a novel SOX2:MYC signaling axis and provide important insights into the molecular mechanisms through which SOX2 elevation induces a slowly cycling proliferative state.